1. Field of the Invention
This invention relates to semiconducting porcelain having a positive temperature coefficient of resistance, of which the electric resistance greatly increases at a temperature exceeding the Curie point. More particularly, it is directed to anti-reducing semiconducting porcelain which is principally employed to make heaters of the automatic termperature control type, temperature sensors, and other devices used in a reducing gas atmosphere.
2. Description of the Prior Art
It is known that the porcelain produced by firing a mixture of barium titanate with a rare earth element, such as Y, La, Sm, Ce or Ga, or a transition element, such as Nb or Ta, at a temperature of 1200.degree. C. to 1400.degree. C. in the air has a positive temperature coefficient of resistance, i.e., increases its electric resistance abruptly at the Curie point. This nature endures such porcelain useful for making heaters, temperature sensors, etc.
It is, however, also known that the semiconductor devices which are produced from positive coefficient semiconducting porcelain composed mainly of a barium titanate semiconductor have a lower positive temperature coefficient of resistance if they are used in a reducing atmosphere, such as an atmosphere containing hydrogen gas or gasoline. If such a semiconductor device is, for example, used in a heater of the automatic temperature control type, a lowering in its positive temperature coefficient of resistance results in the failure of its resistance to increase despite an elevation of its temperature to a predetermined level, and leads in the worst case to its damage by an electric current passing therethrough. It is also known that such a lowering in positive temperature coefficient of resistance occurs not only in a reducing atmosphere, but also in a neutral atmosphere, such as nitrogen or argon, even though to a smaller extent.
It has, therefore, been possible to use those semiconductor devices only in a limited environment. In the event such a device is used in a reducing atmosphere, it has been necessary to confine it in a plastic or metallic case to isolate it from its enviroment. The case prevents the device from releasing heat satisfactorily and working efficiently. The case adds to the number of the parts and the number of the manufacturing steps with a resultant increase of manufacturing cost.